This invention is related to permeable bandages and magnetic cores for rotors of rotating electrical machines. The proposed method could be especially useful for high-speed electrical machines.
The proposed method could be used for containment of permanent magnets, for containment of magnetic cores with permanent magnets, for containment of metal squirrel cages of high speed induction machines, for containment of electrical windings installed in the slots of the rotor, for closing rotor slots or even as a mean of assembly of complex rotors consisting of separate parts or sectors through compressing and containing these parts or sectors against a shaft.
Permanent magnets located on rotors of electrical machines often require containment sleeves. Such containment sleeves can be performed out of metal as described in US 2008/0238234 A1 and U.S. Pat. No. 7,042,118. Solid sleeves provide a path for eddy currents, which increases the rotor temperature. In US 2009/0261678 A1 segmentation of the metal containment bandage is suggested. This however does not exclude eddy currents completely. With respect to assembly of such a rotor, it becomes more complex, since each ring has to be mounted using thermal expansion. The utilized mounting procedure also requires very tight tolerances. Because of eddy currents and increased leakage of permanent magnets it is not practical to use permeable metals or alloys in solid or segmented containment metal bandages. So either Inconel or other non-magnetic materials are typically used in such bandages. This increases the non-magnetic space between the magnets and the inner surface of the stator. So in order to achieve required flux density in the air-gap of such a machine an additional volume of permanent magnets would be required.
It is also possible to use composite sleeves preferably based on carbon fibers, since these materials are strong and light. Use of such bandages is suggested, for instance, in U.S. Pat. No. 6,459,185. However use of composites also leads to some problems. The composites are normally electrically non-conducting or possess high resistivity if carbon fiber is used. This means that practically no electrical currents would be generated in a composite sleeve. But this also means that no shielding would be present for the magnets against pulsations of magnetic field in the air-gap due to typically slotted stator structure and/or because of current pulsations in the stator winding. Composites are also non-magnetic. This means that the non-magnetic space between the magnets on the rotor and the inner surface of the stator is equal to the sum of the thickness of the mechanical air-gap and the composite sleeve thickness. In order to force the main magnetic flux of the machine through this non-magnetic space and maintain required level of the flux density in the air-gap a larger volume of permanent magnets would be required, compared to a situation when sleeve is manufactured out of a permeable material. A larger volume of the magnets obviously requires a thicker containment sleeve. This increases the cost of the machine and decreases the resonance frequency of the rotor, which eventually restricts the achievable operational speed of the machine. Composite sleeves also have fairly poor radial thermal conductivity. So a larger cooling flow would be required in order to cool the rotor. Generally speaking, composite sleeves put a limit on achievable power density in high speed permanent magnet machines.
In high-speed induction machines there is a need to contain a conducting preferably copper squirrel cage, because of very high centrifugal forces. Since induction machines are very sensitive to the value of the air-gap, the containment bandage should preferably be permeable, eddy-current free and capable of sustaining large operational temperatures. Since neither non-magnetic metal containment cylinders described above, nor composite bandages satisfy these requirements, no squirrel cage is typically used in high-speed induction machines. This adversely affects performance of high-speed induction machines.
Some machines utilize rotors with an electric winding, like, for instance, synchronous machines and DC machines. The rotor winding is located in slots of a magnetic core of the rotor. These slots are often made open or half-open in order to facilitate insertion of the winding into the slots. However such a slotted rotor structure can have a negative effect on the machine performance. A permeable bandage around the rotor would smoothen the effect of slots on either side of the air-gap of the machine.
High speed machines often utilize active magnetic bearings. Magnetic bearings exert magnetic forces in order to maintain position of the rotor. Good dynamic operation of magnetic bearings requires an eddy-current free magnetic core on the rotor. This requirement is important, because eddy currents dampen rapid changes of magnetic flux in the magnetic circuit of the bearing, reduce operational frequency range and generally deteriorate performance of such a bearing. Conventional laminated cores are suitable for active radial magnetic bearings. However such configuration is not suitable for alternating axial magnetic flux. There are also other applications requiring 3D eddy-current free permeable cylindrical magnetic cores.